Laser-Based Spherical Coordinate Measurement Systems, more commonly known in the art as Laser Trackers, were introduced in the mid 1980s. See U.S. Pat. No. 4,714,339 Three and Five Axis Laser Tracking Systems to Lau and Hocken; U.S. Pat. No. 5,764,360 Electro-Optical Measuring Device for Absolute Distances to Meier; and U.S. Pat. No. 7,352,446 Absolute Distance Meter that Measures a Moving Retroreflector to Bridges and Hoffer; all three of which are incorporated by reference herein.
Earlier instruments were limited to laser interferometers, which integrated length measurements in the radial direction by fringe counting. Later instruments include absolute distance measurement, which allows the beam to switch between a plurality of targets, or for the beam to be broken. See Large-Scale Metrology—An Update Estler, Edmundson, Peggs, and Parker, Annals of the CIRP, Vol. 51/2/2002, which is incorporated by reference herein, for a review of the technology.
These instruments are calibrated under The American Society of Mechanical Engineers Standard ASME B89.4.19-2006, incorporated by reference herein. The Foreward to the Standard states;                Performance evaluation of a laser tracker presents challenges different from those associated with conventional Cartesian CMMs. Because of the very large working volume, no full-scale three-dimensional calibrated artifacts exist, and the design of the laser beam steering system is such that individual parametric errors cannot, in general, be isolated and measured individually. For any coordinate measurement system, a fundamental requirement is a test of its ability to realize the SI unit of length, the meter. In a laser tracker, the length scale is often a laser interferometer and usually one does not have a significantly more accurate reference interferometer with which to perform such a test.        For these reasons, the performance evaluation tests in this Standard consist primarily of point-to-point length measurements using calibrated artifacts that can be realized in a number of ways. Measured lengths are compared with manufacturers Maximum Permissible Error (MPE) specifications in order to decide conformance. Realization of the SI meter can be evaluated in a number of ways, including calibration of the laser interferometer, measurement of a series of short calibrated reference lengths, or measurement of a series of long calibrated reference lengths. Procedures are also included for testing the absolute distance measurement (ADM) capability of laser trackers that include this option.        